ABSTRACT Mycoplasma genitalium (MG) is an important reproductive tract pathogen associated with nongonoccocal urethritis (NGU), cervicitis, endometritis, pelvic inflammatory disease, preterm birth, and tubal factor infertility, and notable for its extremely reduced genome. MG is more prevalent than Neisseria gonorrhoeae in many settings and is a risk factor for HIV acquisition. MG may persist for months to years in infected individuals, increasing the risk of transmission and the potential for serious reproductive tract sequelae. These findings highlight the urgency of understanding the molecular pathogenesis of this novel pathogen while the emergence of antibiotic resistance and threat of untreatable strains of MG underscore the need to devise alternative prevention and treatment regimens for this underappreciated and underfunded pathogen. Over the past decade our laboratory has focused on the variability and antigenicity of two key proteins required for MG pathogenesis, the adhesins MgpB and MgpC. This research has demonstrated that MgpB and its accessory protein, MgpC, undergo phase and antigenic variation via a unique system of homologous reciprocal recombination between the mgpBC expression site and partial copies archived in the chromosome. Although MgpBC antigenic variation may allow MG persistence and re-infection, paradoxically the non-variable C- terminus contains both the adhesin domain and the dominant epitopes targeted by antibody-mediated complement killing. It is imperative to understand how MG persists in the face of a pronounced antibody response that targets a highly conserved region of the immunodominant adhesin. To this end, the experiments proposed will characterize the interactions between bacteria and host cell receptors, in addition to more accurately defining and comparing the epitopes within the C-terminus responsible for attachment and/or antibody-directed complement killing. This proposal utilizes an innovative combination of experimental techniques including Surface Plasmon Resonance to investigate the thermodynamics and kinetics of MgpB- receptor and MgpB-antibody interactions in experimentally infected/immunized animals and persistently infected men in addition to those techniques that have been historically successful in Mycoplasma research including hemadsorption, hemagglutination, and complement-mediated killing. The identification of biologically active immunogenic epitopes and the localization of functional domain(s) within the conserved C-terminus of MgpB are key to understanding the pathogenesis of MG infection. The data generated by this application, along with approaches that pioneer new experimental protocols in the difficult field of Mycoplasmology, will lead to future studies developing novel MG treatment and prevention strategies, including vaccines, to combat this increasingly antibiotic-resistant and important STD pathogen.